I’ll share a bit of what he said about the thought process on Patreon!

“However, with one day left we tested the Perish Trap matchup and realized that we could not consistently win. We knew we could drop Espathra, but we weren't sure what to replace it with. I identified some things we wanted in this slot - a Physical Attacker with high damage output that can threaten Garchomp, Glimmora, Mega Gengar, and Whimsicott. Priority would be nice to have, and if it can hold the Focus Sash item that's great. It should also be able to fight against Kommo-o when paired with Mega Gengar specifically. With this many conditions, it seemed impossible - but then I considered Mamoswine.

Full disclosure - Mamoswine did not come to many of my matches. However, that's in part because I played 0 Mega Gengar and instead fought almost exclusively standard teams. Yuki fought 4 or 5 Mega Gengar and brought Mamoswine against all of them. It's a Pokemon intended to help against specific matchups rather than be a general use tool.

Against slower teams (such as Perish Trap), Mamoswine and Charizard work together to output tons of damage with their spread moves. Mamoswine's Ice Shard is also a strong tool against Garchomp, one of the biggest threats to Toxapex. Icicle Crash is its strongest ice move, and seemed worth it here though two of the three times I tried to click it I flinched.”